Method of making vulcanisable rubber compositions

Abstract: The invention relates to rubber compositions particularly suitable for tyre treads having an improved balance of properties such as rolling resistance, grip and wear. It provides a vulcanisable rubber composition which is obtained by mechanical blending of a rubbery polymer, vulcanisation agent, filler and coupling agent, the filler comprising silica and the coupling agent being at least bi-functional whereby it can react with the silica and with the polymer, the temperature of the composition being maintained below 130°C during the mechanical blending stage. The invention also provides a method of making such a vulcanisable rubber composition and a tyre having a tread formed by the vulcanisation of such a rubber composition.

Description: This invention relates to a method of making vulcanisable rubber compositions, e.g. for the manufacture of tyre treads. It is well known that it is desirable to reduce the rolling resistance of a motor vehicle tyre in order to achieve reduced fuel consumption. However, it is also well known that reductions in rolling resistance are normally achieved at a cost, namely reduced tyre grip and/or reduced wear resistance. Previous proposals to reduce the rolling resistance of a tyre have included reducing the amount of carbon black in the head composition, using a carbon black of relatively low surface area and adding a polymer with a relatively low glass transition temperature. None of these proposals has been entirely successful since, for example, a typical consequence is an undesirable reduction in tyre wet grip. In EP-A-0547344 is disclosed a vulcanisable rubber composition comprising a non-conjugated ethylene-propylene-diene rubber with a cross-linker, an alkoxysilane and a silica or siicate filler in which mixing and vulcanisation temperatures disclosed are conventional. US-A-4430466 discloses a sulphur-vulcanisable diene rubber composition including a siliceous filler, silane coupling agent and an accelerator in which the temperatures reached after blending the ingredients are from 150° to 160°C. AT-B-392,795 also discloses a mixture of cross-linkable nibber, sulphur, silane coupling agent and silica filler, again with a conventional blending stage. EP-A-0028911 discloses a vulcanisable rubber composition including silane, silica and carbon black that is particularly intended for use in a belt. Again its teaching of the blending stage is conventional. There remains a need to improve tyre tread compositions to achieve a better overall balance of properties, in particular to improve, i.e. reduce, rolling resistance without impairing other properties such as the above-mentioned grip and wear. Accordingly, the invention provides a method of making a vulcanisable rubber composition in which a rubbery polymer is mechanically blended with filler, vulcanisation agent and coupling agent, the filler comprising silica and the coupling agent being at least bifunctional to react with the silica and with the polymer, characterised in that the temperature of the composition during said blending is maintained below 130°C. The coupling agent may be pre-coupled with the silica prior to the mechanical blending stage or it may couple in situ during the mechanical blending stage and/or a later vulcanisation stage. Normally the reaction between the coupling agent and the polymer will be during the vulcanisation stage when the composition is heated to vulcanisation temperature. In a preferred embodiment the mechanical blending of the composition is carried out at a temperature of 125°C. It may be carried out at temperatures as low as 100°C or lower, but a temperature of at least 110°C is preferred. The coupling agent is preferably a silane coupling agent, e.g. bis(3-triethoxy-silyl propyl)-tetrasulphide or mercapto propyl triethoxy silane, particularly where the vulcanisation agent is sulphur. In these examples the alkoxy moieties react and couple with the silica and the sulphide moieties react with the rubbery polymer when the composition is vulcanised. Alternatively, the coupling agent may be, for example, a vinyl silane, e.g. vinyl triethoxy silane, or a thiocyanato triethoxy silane, e.g. thiocyanato-propyl-triethoxy silane, or a zirconate coupling agent, e.g. zirconium dineoalkanato di(3-mercapto) propionato-0. The silica preferably has a nitrogen surface area of from 60 to 300 m 2 /g especially 80 to 250m 2 /g, and is preferably a precipitated silica. When precoupled it is treated with the coupling agent prior to its incorporation in the rubber composition by, for example, physical mixing at ambient temperature and then subjecting the mixture to elevated temperature to promote the coupling reaction. The composition may contain silica as the only filler or, if desired, admixtures with other fillers, e.g. carbon black, may be used. Where silica is the only filler, it is preferably used in an amount of from 30 parts to 120 parts per hundred parts by weight of rubbery polymer. Where other fillers are included the silica is preferably used in all amount of at least 15 parts per hundred parts by weight of rubbery polymer and the total filler content may preferably be up to 120 parts by weight. The rubber composition may contain conventional additives, e.g. accelerators, activators, extenders and anti-degradants, as desired. The vulcanisation agent is preferably sulphur or a sulphur-containing compound but any other suitable vulcanisation agent may be used e.g. phenolic resin or peroxide. The vulcanisation agent may be added to the composition later in the mixing stage after the initial mechanical mixing together of the other ingredients or, preferably, it may be added in a subsequent mixing stage. The rubbery polymer may be any polymer suitable for use in a tyre tread or in any other component of a tyre where the property of low rolling resistance is of benefit, for example in the undertread or sidewall. For example, it may be a solution- or emulsion-polymerised styrene-butadiene copolymer, a polybutadiene homopolymer or a vinyl polybutadiene or it may be a blend of polymers. The coupling agent may be used in an amount of from 2 to 15 per cent by weight of the silica filler, preferably from 5 to 12 per cent. The invention is particularly suitable for the provision of a tyre tread formed from a vulcanised composition which has been obtained by the method of the invention, i.e. by mechanical blending of a rubbery polymer, filler, vulcanisation agent and coupling agent, the filler comprising silica and the coupling agent being at least bi-functional to react with the silica and with the polymer, wherein the temperature of the composition being maintained below 130°C during the mechanical blending stage and then vulcanising the blended composition. We have surprisingly found that the use of silica as a filler together with a coupling agent in tyre tread compositions together with the use of relatively low temperatures in the mechanical blending or compounding stage to mix the ingredients have beneficial effects on the properties obtained. Thus vulcanised tyre tread compositions of the invention have the desired reduced rolling resistance compared with conventional tyre treads while retaining an acceptable balance of grip and wear resistance. Moreover, this is achieved in an energy-saving manner because of the relatively low temperature compounding stage. In order to demonstrate rolling resistance and grip properties of tyre tread compositions it is conventional practice to subject the compositions under laboratory conditions to viscoelastic testing and to predict the properties from the hysteresis characteristics displayed in the tests. The tests subject the tread composition to sinusoidal dynamic deformation under shear, tension or compression at a controlled temperature. It is well known for the wet grip and rolling resistance to be interpreted from the loss modulus and loss compliance values at temperatures of typically 0°C and 50°C respectively. Thus for high wet grip a high loss modulus and/or a high loss compliance at 0°C is desirable, whilst for low rolling resistance a low loss modulus and/or loss compliance at 50°C is desirable. Thus the invention will now be further described by way of example only with reference to the following specific compounding and testing procedures and the Examples as detailed below. Tyre tread compositions as detailed below were prepared using a 2-stage mixing procedure. In the first stage the compositions were each mixed in a 2 litre laboratory internal mixer up to a discharge temperature and for a mixing time as indicated below. In the second stage the mixing was completed by the addition of the sulphur and accelerator on a laboratory two roll mill at about 50°C for 5 minutes. The mixed compositions were then cured in moulds for 20 minutes at 165°C. Viscoelastic testing of the cured samples was then conducted as follows:- Pre-tension 10% Dynamic Strain 0.5% (Double Strain Amplitude) Frequency 10Hz Parameters Complex Modulus E* Dynamic Modulus E 1 Loss Modulus E 2 Loss Tangent (TD) E 2 /E 1 Loss Compliance (LC) E 2 /(E*) 2 Abrasion testing was conducted according to DIN 53516. The compositions used, the mixing details and the test results obtained were as follows:- Examples 1, 2 and 3 Parts by Weight 1 2 3 SBR(A) 50.0 50.0 50.0 SBR(B) 68.8 68.8 68.8 N220 Carbon Black 68.0 - - Silica - 68.0 68.0 Coupling Agent - 11.0 11.0 Aromatic Oil (D) 12.4 12.4 12.4 Zinc Oxide 2.0 2.0 2.0 Stearic Acid 1.5 1.5 1.5 6PPD 1.0 1.0 1.0 Hydrocarbon Wax 1.5 1.5 1.5 Sulphur 1.8 1.8 1.8 CBS (accelerator) 1.3 1.3 1.3 DPG (accelerator) - 2.0 2.0 Footnote SBR(A) = Solution SBR, 15% styrene. SBR(B) = Solution SBR, 30% styrene, extended with 37.5 parts aromatic oil. Silica = Precipitated silica surface area 175m 2 /g marketed as Ultrasil VN3 by Degussa AG, Frankfurt, Germany. ("Ultrasil" is a Registered Trade Mark.). Coupling Agent = Bis(3-triethoxysilylpropyl)-tetrasulphide, admixed 50:50 with N330 carbon black. Marketed as X-50S by Degussa AG (Frankfurt, Germany). 6PPD = N(1,3-dimethylbutyl)-N 1 -phenyl-p-phenylene diamine. CBS = N-Cyclohexyl benzthiazole sulphenamide. DPG = Diphenylguanidine. Aromatic Oil (D) = S.G. 0.97 and viscosity (100°C) of 55 mm 2 /sec (cst). Mixing details (First Stage) 1 2 3 Discharge Temperature (°C) 125 170 125 Mixing Time (mins) 5.5 7.5 5.5 Viscoelastic Testing 1 2 3 0°C E* MPa 36.0 17.0 34.0 E 2 MPa 9.7 5.6 9.9 TD 0.28 0.34 0.31 LC (GPa) -1 7.6 18.8 8.8 50°C E* MPa 12.5 7.6 12.7 E 2 MPa 2.1 0.7 1.4 TD 0.17 0.09 0.11 LC (GPa) -1 13.0 12.0 8.7 Abrasion Testing Volume Loss mm 3 106 85 109 It will be seen from the above results that the replacement of carbon black with silica and coupling agent (Examples 2 and 3) provides a reduction in loss modulus (E 2 ) and loss compliance (LC) at 50°C. Rolling resistance of tyre treads of Examples 2 and 3 will, therefore, be reduced. Unexpectedly, mixing silica at the lower temperature of 125°C (3) resulted in a beneficial maintenance of dynamic modulus (E*) compared with the high temperature mix (2). This will permit tyre handling properties to be maintained at the level of the carbon black compound (1). The lower temperature silica mix also provides a more satisfactory balance of E 2 and LC at 0°C, which is to the benefit of wet grip. Wear resistance of the composition of Example 3 is maintained at a level comparable with that of the carbon black compound. Examples 4 and 5 The following further examples were carried out to show the improvement of the invention where the polymer used is an emulsion SBR. Compounding and testing conditions were as for the previous examples except if stated otherwise. Parts by Weight 4 5 SBR (C) 100.0 100.0 N234 Black 75.0 Silica - 75.0 Coupling Agent - 12.0 Aromatic Oil (D) 56.0 56.0 Zinc Oxide 3.0 3.0 Stearic Acid 1.0 1.0 6PPD 2.0 2.0 Hydrocarbon Wax 1.0 1.0 Sulphur 1.0 1.0 CBS 2.6 2.6 FOOTNOTE SBR (C) = Emulsion SBR, 40% styrene, extended with 37.5 parts aromatic oil. Aromatic Oil (D) = SG 0.97, viscosity (100°C) 55 mm 2 /sec (cst). Mixing Details 4 5 Discharge Temperature (°C) 125 125 Mixing Time (mins) 5.5 5.5 Viscoelastic Testing 4 5 0°C E* MPa 34.0 21.0 E 2 MPa 12.5 9.6 TD 0.39 0.51 LC (GPa) -1 10.8 21.2 50°C E* MPa 5.9 5.3 E 2 MPa 1.4 0.83 TD 0.24 0.16 LC (GPa) -1 39.2 30.1 It will be seen from the above results that the replacement of carbon black with silica and coupling agent at a mixing temperature below 130°C gives an increase in Loss Compliance at 0°C (for improved wet grip) and a reduction in Loss Modulus and Loss Compliance at 50°C (for lower rolling resistance). The dynamic modulus at 50°C is also maintained at a level comparable with that of the carbon black compound, for acceptable tyre handling properties.

Claims: A method of making a vulcanisable rubber composition in which a rubbery polymer is mechanically blended with filler, vulcanisation agent and coupling agent, the filler comprising silica and the coupling agent being at least bi-functional to react with the silica and with the polymer, characterised in that the temperature of the composition during said blending is maintained below 130°C. A method according to Claim 1, wherein the coupling agent is precoupled with the silica prior to the mechanical blending stage. A method according to Claim 2, wherein the coupling agent is precoupled with the silica by physically mixing them at ambient temperature and then subjecting the mixture to elevated temperature to promote the coupling reaction. A method according to Claim 1, wherein the coupling agent is coupled with the silica during the mechanical blending stage and/or a later vulcanisation stage. A method according to any preceding claim, wherein the reaction between the coupling agent and the polymer takes place when the composition is heated to vulcanisation temperature. A method according to any preceding claim, wherein the mechanical blending of the composition is carried out at a temperature of at least 110°C. A method according to any preceding claim, wherein the vulcanisation agent is added to the composition in a subsequent stage after mixing of other ingredients. A method according to any preceding claim, wherein the coupling agent used is a silane coupling agent. A method according to Claim 8, wherein the coupling agent used is bis(3-triethoxy-silyl propyl) tetrasulphide or mercapto propyl methoxy silane. A method according to Claim 8, wherein the coupling agent used is a vinyl silane or a thiocyanato-triethoxy silane. A method according to Claim 10, wherein the coupling agent used is vinyl triethoxy silane or thiocyanato-propyl-triethoxy silane. A method according to any one of Claims 1 to 7, wherein the coupling agent used is a zirconate. A method according to Claims 12, wherein the coupling agent is zirconium dineoalkanato di(3-mercapto) propionato-0. A method according to any preceding claim, wherein the silica used is a precipitated silica. A method according to any preceding claim, wherein the silica used has a nitrogen surface area of from 60 to 300 m 2 /g. A method according to Claim 15, wherein the silica used has a nitrogen surface area of from 80 to 250 m 2 /g. A method according to any preceding claim, wherein the silica used is the only filler and is used in an amount of from 30 to 120 parts per 100 parts by weight of the rubbery polymer. A method according to any one of Claims 1 to 16, wherein other material filler is used in addition to the silica and the silica filler is used in an amount of at least 15 parts per hundred parts by weight of the rubbery polymer and the total filler content used is up to 120 parts by weight. A method according to any preceding claim, wherein the vulcanisation agent used is sulphur, a sulphur-containing compound, phenolic resin or peroxide. A method according to any preceding claim, wherein the rubbery polymer used is a solution - or an emulsion-polymerised styrene-butadiene copolymer, a polybutadiene homopolymer or a vinyl polybutadiene. A method according to any preceding claim wherein the coupling agent is used in an amount of from 2 to 15 per cent by weight of the silica. A method according to Claim 21, wherein the coupling agent is used in an amount of from 5 to 12 per cent by weight of the silica.

Inventors: POWELL BRIAN DAVID WILLIAM, POWELL, BRIAN DAVID WILLIAM
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1994
Main classification: C08L9
IPC codes: C08K9, C08K3